Dehydrohalogenation of an
Alkyl Halide
Preparation and Characterization of
an Alkene Mixture
DID SOMEONE SAY VICTORY?
HOW ABOUT THAT GAME????
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Dehydrohalogenation
Alkenes are most often prepared by
dehydration of alcohols or by
dehydrohalogenation of alkyl halides
H
OH
H-Z
H
E1
H
:Y
E2
X
Dehydrohalogenation of
3-Chloro-3,7-dimethyloctane
The alkyl halide we are using today is
a 3o alkyl chloride, so SN2 substitution
is very unlikely.
X:
3O
Cl
SN2
2
Dehydrohalogenation of
3-Chloro-3,7-dimethyloctane
In an E2 elimination, the transition state
requires the C-H bond to be anti-periplanar
(180o dihedral angle) to the C-X bond.
H
:Y
+ HY + X:
E2
X
Reaction is faster when X is a better leaving
group (I > Br > Cl )
Dehydrohalogenation of
3-Chloro-3,7-dimethyloctane
In 3-chloro-3,7-dimethyloctane, we
have 3 different sets of C-H bond that
can react in the E2 reaction.
Hb
Hb Hc
Ha
Ha
Hc
Cl
Ha
3
Dehydrohalogenation of
3-Chloro-3,7-dimethyloctane
Elimination involving any of the three
Ha protons will give the same product.
Hb
H b Hc
Ha
Hc
Cl
Ha
CH2
Ha
2-ethyl-6-methylheptane
Dehydrohalogenation of
3-Chloro-3,7-dimethyloctane
However, elimination of an Hb proton
can give either of two diastereomeric
products.
Hb
Hb Hc
Hc
(E)-3,7-dimethyloct-2-ene
Ha
Ha
Cl
Ha
(Z)-3,7-dimethyloct-2-ene
4
Dehydrohalogenation of
3-Chloro-3,7-dimethyloctane
CH3
Hb2
Y:
Cl
CH2R
CH3
Hb1
CH3
CH2R
+ HY + Cl
E2
H
CH3
(Z)
Hb1
H3C
Y:
Cl
CH2R
CH3
Hb2
H
CH2R
+ HY + Cl
E2
H3C
CH3
(E)
Dehydrohalogenation of
3-Chloro-3,7-dimethyloctane
Likewise, elimination of one of the Hc
protons can give two different
diastereomeric products.
Hb
Hb H c
Hc
(E)-3,7-dimethyloct-3-ene
Ha
Ha
Cl
Ha
(Z)-3,7-dimethyloct-3-ene
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Procedure
Weigh 1.32g of your stored 3-
chloro-3,7-dimethyloctane into
a micro round-bottomed flask.
Add 6mL of 95% ethanol, 1.0
mL of 45% KOH, and 3-4
boiling chips.
Connect a Claisen head and air
condenser. Be sure to grease all
joints!
Procedure
Use the heating mantle to
heat the mixture vigorously
at reflux for 20 min.
Note: Reflux means to heat
so that the vapors formed
condense and return to be
heated again.
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Procedure
You need to start counting toward 20
minutes ONLY after the solution is
refluxing vigorously. NOT BEFORE.
Use heat shield (porcelain) on your
heating mantles. Turn the heat up to 35
on the Variacs to start reflux and
reduce to about 30 after reflux starts.
EXTRACTION - IMPORTANT
Reread the section on pages 67 and 68 in the
lab manual about extraction in test tubes.
In your extractions today, the organic layer
(the layer to save) will ALWAYS be the
upper layer. Do NOT use a lower layer as
your sample layer.
If you use an aqueous layer as your product
layer, you will have NO alkene and your
qualitative tests will all be negative.
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EXTRACTION
After the 20 minutes of reflux, pour the
mixture into a test tube containing 15 mL of
water. Let the layers separate, and transfer
all of the upper organic layer to another test
tube containing 15 mL of water. Mix and
let the layers separate.
Procedure (Cont.)
EMULSION! If the layers don’t separate
well, obtain 1 – 2 mL of concentrated
sodium chloride solution from your TA,
add to the tube, and shake again.
Remove the lower aqueous layer and
wash the organic layer 3 times with 3-mL
portions of water. Remove as much
water as possible in the last wash.
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Procedure (Cont.)
Transfer the organic product to a
sample vial and add 0.5 g of calcium
chloride to dry the product. There must
be a single layer with no visible water
droplets.
IMPORTANT NOTE: Do NOT leave
the cap off of the CaCl2 bottle! Keep it
capped; it is very hygroscopic!
Procedure (Cont.)
Filter the product through a small plug of
cotton in a pipet into a preweighed vial.
Weigh the vial to obtain a yield.
Perform a bromine test and a permanganate
test on small amounts of the product.
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PROCEDURES FOR ANALYSIS
You MUST perform the qualitative tests for
alkenes and take an IR spectrum BEFORE you
will be allowed to do a GC analysis on your
product!
You will be allowed to do a GC analysis
ONLY IF your qualitative tests are positive and
your IR does not show excessive –OH peaks.
If the tests are negative you have NO alkene to
analyze. Your IR will likely confirm the lack
of organic peaks and presence of water peaks.
SAFETY
Be very careful with the ethanolic solution
of KOH.
Allow glassware to cool before
disassembling.
Potassium permanganate, bromine and
bromine vapor are hazardous.
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